Microlithographic projection exposure apparatuses are used to transfer structures which are contained in a mask or arranged thereon to a resist or another light-sensitive layer. Optical components of a projection exposure apparatus include a light source, an illumination system which prepares projection light generated by the light source and directs it onto the mask, and a projection lens, which images the region of the mask illuminated by the illumination system onto the light-sensitive layer.
In general, the shorter the wavelength of the projection light is, the smaller the structures are that can be defined on the light-sensitive layer with the aid of the projection exposure apparatus. The most recent generation of projection exposure apparatuses uses projection light with a mean wavelength of approximately 13.5 nm, which therefore is situated in the extreme ultraviolet spectral range (EUV). Such apparatuses are often referred to as EUV projection exposure apparatuses for short.
However, generally, there are no optical materials which have a sufficiently high transmissivity for such short wavelengths. Thus, the lenses and other refractive optical elements, which are conventional at longer wavelengths, have been replaced by mirrors in EUV projection exposure apparatuses, and the mask too therefore contains a pattern of reflecting structures.
The provision of mirrors for EUV projection exposure apparatuses can be a great technological challenge. Coatings that are suitable for EUV light and applied to a mirror substrate often include more than 30 or 40 double layers with a thickness of only a few nanometers, which are applied onto one another by vapor deposition in technologically complex processes. Even in the case of coatings with such a complex design, the reflectivity of the mirrors for the EUV light is usually hardly more than 70%, and, in general, this also only holds for light that impinges perpendicularly onto the reflective coating or with angles of incidence of a few degrees.
The upshot of the comparatively low reflectivity of the mirrors is that an effort has been made during the development of projection exposure apparatuses to use as few mirrors as possible because each mirror means a loss of light and ultimately reduces the throughput of the projection exposure apparatus.
However, the relatively low reflectivity of the mirrors is often also accompanied by thermal issues because the portion of the energy-rich EUV light that is not reflected by the coating can be absorbed and lead to a temperature increase in the mirrors. In general, the heat generated in the process is substantially dissipated by thermal conduction via the mirror substrate because projection exposure apparatuses are usually operated in a vacuum on account of the high absorption of EUV light by gasses and therefore convective heat dissipation by gas cooling is ruled out.
So that occurring temperature gradients in the mirror substrates do not lead to an undesired deformation of the mirrors, the use of materials which have a small (even vanishingly small) thermal expansion coefficient at the operating temperature is often favored for the mirror substrates. Such glass-based materials are distributed by e.g. Schott under the branding Zerodur® and by Corning under the branding ULE®. As a result of additional measures, thermal deformations which are caused by EUV light absorption can be kept low or at least the effects thereof on the optical properties of the projection lens can be kept within tolerable limits.
Thus, U.S. Pat. No. 7,477,355 B2 proposes to heat mirrors with the aid of an additional heating mechanism such that a temperature is set in the substrate material of the mirrors at which the thermal expansion coefficient equals zero or at least is minimal. Temperature variations during the operation of the apparatus then have no effect, or only a small effect, on the imaging properties of the mirror.
U.S. Pat. No. 7,557,902 B2 describes a projection lens in which two mirrors contain materials, the thermal expansion coefficient of which increases with increasing temperature in one of the two mirrors and decreases with increasing temperature in the other mirror. What this can achieve in the case of a suitable selection of the mirrors is that although the two mirrors significantly deform in the case of a temperature change, the optical effects of these deformations largely cancel out one another.
DE 103 17 662 A1 discloses an EUV projection exposure apparatus with a heating light source, which illuminates selected regions on imaging mirrors with additional heating light. The absorption of the heating light leads to an at least approximately homogeneous temperature distribution on the surface of the mirror. Here, a suitable configuration makes it possible to achieve a temperature set in thermal equilibrium, at which the thermal expansion coefficient of the mirror substrate has a minimum value. As a result, relatively small temperature variations or remaining inhomogeneities in the temperature distribution can no longer lead to noteworthy thermal deformations of the mirror substrate, and hence to aberrations.
In order only to illuminate selected regions on the mirror with the heating light, a transmission filter is arranged behind the heating light source of this known projection exposure apparatus, the filter shadowing those regions on the mirror surface on which projection light is incident and which should therefore not be additionally heated by heating light. If the heating light should form a pattern with as sharp edges as possible on the mirror surface, provision can be made for an additional imaging optical system which images the transmission filter onto the mirror surface. In another exemplary embodiment, a laser which is associated with a controllable ray-deflection device is used as heating light source. This ray-deflection device is used to direct the laser ray generated by the laser onto the desired regions on the mirror surface only. The ray-deflection device can in this case include rotating mirrors, as are contained in a similar fashion in barcode scanners which are known.
A disadvantage of this known projection exposure apparatus can be that when a plurality of mirrors are illuminated with heating light, a heating light source and optical components are provided for each of these mirrors. These components are arranged such that they direct the heating light onto the respective mirror at an angle of incidence that is not too shallow. However, there often is not enough installation space available in the projection lens for such optical components and also for the heating light sources.